I'm confused.  Are there any differences between the Z110 through the z150 or is it all just software?  The reason I ask is because I have seen several threads about upgrading one system to the next.  Related question which model is the cheapest/best to buy used?

Yes, there's a big difference between the models.

The Z150 was the top of the line, a few years ago.

The new line is the NEX Series.

Just depends on what features you are after.

You can find a Z150 on eBay for around $300-400.